It is believed that the first re-usable plastic drive rivets were those disclosed by our parent company in their British Pat. No. 2,051,288.
In accordance with those rivets, amongst other necessary features, a drive pin is associated with a headed shank, and is receivable within a bore extending axially through both the head and the shank. The intention is that the shank is passed through aligned apertures defined within respective workpieces until the head of the shank abuts one side of the workpieces, the drive pin then being forced into the bore by means of an axially directed blow so as to expand the shank against the other side of the workpieces. To facilitate such expansion, the shank is split along its length and the bore is narrowed so that the shank is forced radially outwardly as the drive pin is driven axially therethrough. The re-usability is achieved by providing thread means upon the drive pin and interrupted complementary thread means within the bore. Other design features ensure that the drive pin is drivable axially into the bore without the bore thread means being sheared off by the drive pin thread means, thereby permitting the drive pin to be subsequently removed from the bore by means of an unscrewing operation.
A disadvantage of the aforenoted rivets, particularly in the usual situation where the shank presents peripherally extending shoulders which are deflected inwardly as they are passed through the workpiece apertures before returning to their initial configuration with a snap action as they emerge from the workpiece apertures, is that the shank can be difficult to remove even after the drive pin has been removed from the shank.
In practice, a flat bladed screw driver can be inserted between the head of the shank and the adjacent workpiece so as to pry the shank out of the adjacent workpiece. This procedure often results in scratch marks being generated upon the adjacent workpiece in locations which are not hidden by means of the head when the rivet is re-used. If the adjacent workpiece is formed of a cloth-like material, it can easily be torn while removing the shank therefrom.
A particular situation in which this difficulty has been encountered has been when using rivets to fix a roof lining to a supporting panel for the roof lining of a vehicle. It is sometimes necessary to remove the roof lining from the supporting panel in order to expose electrical wiring for example, and the rivets must therefore be totally removed from the roof lining and the supporting panel. However, the roof lining is usually formed of pressed fibers with a plastic coating which is very easily ripped or otherwise visibly damaged.
A re-usable plastic drive rivet, in accordance with the present invention, comprises an axially extending shank in association with an axially extending drive pin, an outer surface of the shank having thread means thereon and an outer surface of the drive pin being of non-cylindrical outline, the drive pin being drivable axially into a bore which extends axially through the shank and has a non-cylindrical peripheral surface complementary to that of the drive pin.
Preferably, an end of the shank initially adjacent to the drive pin is formed as an enlarged head. In use, the shank is passed through aligned apertures defined within respective workpieces until the head of the shank abuts the workpiece adjacent thereto. In the situation where a vehicle roof lining is to be fixed to a supporting panel, the roof lining (initially disposed closer to the rivet of the present invention) may not have been formed with an aperture until the shank is punched there-through, whereas the supporting panel (initially disposed further from the rivet of the present invention) may already have been formed with a circular or slotted aperture. The drive pin is then forced into the bore of the shank by means of an axially directed blow so as to expand the shank radially outwardly against the workpieces. In particular, as a consequence of driving the drive pin into the bore of the shank, the thread means formed upon the shank is forced radially outwardly into firm contact with at least one of the workpieces, such as the supporting panel for the roof lining.
If it is subsequently necessary to remove the shank from the workpieces, it is merely necessary to apply an appropriate torque to the drive pin in the withdrawal or counterclockwise direction. The torque is transferred from the drive pin to the headed shank, means of the complementary non-cylindrical outlines of the drive pin and the bore wall of the headed shank. The thread means formed upon the shank then, in turn, operate to unscrew the shank from the workpieces.
It is not necessary for the drive pin and the headed shank to be unscrewed with respect to each other only when they have both been completely removed from the workpieces. Once the head of the shank has been moved clear of the adjacent workpiece, it might be possible to pull the drive pin out of the bore of the shank thus removing the expansion force from the shank and permitting easier unscrewing of the shank from the workpiece. The head of the shank may be gripped by means of, a pliers, for example, without damaging the adjacent workpiece once there is sufficient clearance therebetween. Alternatively, the head of the shank may be shaped with flats so as to facilitate the gripping thereof by means of a spanner.
Preferably, the thread means extends substantially the entire length of the shank. The thread means may be one or more helical threads which may have a sharp crest configuration. Preferably, however, in order to provide improved surface contact between the shank and at least one of the workpieces, the thread means has a crest configuration which is substantially flat within a particular section thereof along the length of the shank.
The non-cylindrical outlines of the bore wall and the drive pin may be substantially cylindrical apart from including at least one axially extending discontinuity, in which case the bore wall may include a plurality of the discontinuities in the form of ribs and the drive pin may include a complementary plurality of the discontinuities in the form of grooves, or vice versa, but the bore wall and the drive pin may have polygonal, oval or any other non-cylindrical outlines.
Preferably the end of the drive pin initially disposed remote from the headed shank is itself formed with an enlarged head. The drive pin head may have flat torque transmission means for facilitating the transference thereto of an applied torque. For example, there may be provided hexagonally arranged faces to which a spanner could be applied, or there may be a centrally arranged recess into which a screwdriver could be inserted. The drive pin head may additionally, or alternatively, include peripherally extending sealing means for facilitating circumferentially complete abutment of the drive pin head with the adjacent workpiece, even in the event that there has been a slight mis-alignment between the pin and the shank. For example, the drive pin head may include a resiliently deformable domed skirt whose free edge is intended to press against the adjacent workpiece.
It is possible for the drive pin to be preassembled with the headed shank by initially inserting the drive pin into the bore defined within the headed shank. Particularly in the situation where the drive pin is formed with an enlarged head, there is no need for the shank to be formed with its own enlarged head. In use, if the drive pin is to be fully inserted into the bore of the shank, the leading end of the drive pin may be pointed and project axially beyond the head of the shank so as to assist in the location of the pin in axial alignment with the bore of the shank, and an intermediate part of the drive pin may be undercut so as to facilitate radial inward deformation of the shank during such relative axial operation location, but the manner of operation is otherwise substantially unchanged. It is preferable, however, that the end of the drive pin initially disposed adjacent to the headed shank is integrally formed therewith in coaxial alignment with the bore defined therethrough by means of at least one frangible connection. Naturally, the one or each one of the frangible connections is broken when the drive pin is driven by means of an axially directed blow into the bore of the shank.
The shank may be split at least partly along its length, from its free end remote from its headed end, for use within circular rather than slotted apertures.
A re-usable plastic drive rivet constructed in accordance with the present invention is of special value in situations where a first workpiece which is light in weight is to be secured to a second rigid workpiece. The rigid workpiece may be formed with an aperture of a predetermined width or diameter for optimizing the expansion capability of the thread means formed upon the shank. More generally, the dimensions of the re-usable plastic drive rivet constructed in accordance with the present invention may be chosen in advance so as to be suitable for use with the intended dimensions of the workpieces.